Contamination detection and quantification requirements have become increasingly important, particularly with the rapid evolution of high-tech industries. For example, the semiconductor industry has developed technology for precisely producing microelectronic devices. In order to reliably produce such products, highly stringent contamination standards must be maintained in the production facilities.
In an effort to control and minimize contamination in crucial stages of a production process, “cleanrooms” are frequently used. A cleanroom is a room in which the air filtration, air distribution, utilities, materials of construction, equipment, and operating procedures are specified and regulated to control airborne particle concentrations to meet appropriate airborne particulate cleanliness classifications.
It is important to monitor the cleanliness/contamination levels in a cleanroom, especially for detecting particles on a cleanroom surface. Visual inspection techniques have been used with ultraviolet or oblique white light. Ultraviolet light is employed to take advantage of the fact that certain organic particles fluoresce. Alternatively, white light is shined towards the test surface at an angle so as to produce reflections that can be visualized. While the white light technique is slightly more sensitive than the ultraviolet technique, they both suffer from the same limitations. These visual inspection techniques only allow a cursory inspection of the surface conditions. They do not provide quantitative data. Also, the visual inspection techniques, at best, only detect particles that are larger than twenty microns. It is often desirable to detect particles that are less than one micron.
Another inspection technique involves removing particles from a test surface, by for example, applying a piece of adhesive tape to the test surface. The particles on the tape are then manually quantified by putting the tape under a microscope and visually counting the particles. This technique allows the detection of particles of approximately five microns or larger. The primary disadvantage of this technique is that it is very time consuming, and that it is highly sensitive to variability between operators.
A third inspection technique is disclosed in U.S. Pat. No. 5,253,538. The '538 patent discloses a device that includes a scanner probe having at least one opening for receiving particles from the sample surface. The scanner probe is connected to a tube having first and second ends. The first end of the tube is connected to the scanner probe and the second end of the tube is connected to a particle counter that employs optical laser technology. The particle counter includes a vacuum generator that causes air to flow from the sample surface through the scanner probe, through the tube and into the particle counter, where particles contained in the air stream are counted. The '538 patent discloses an inspection method that involves the use of the particle counting device. A background particle level of zero is first established by holding the scanner probe near the cleanroom supply air and taking repeated readings, or by installing an optional zero-count filter in the particle counter. Next, the hand-held scanner probe is passed over the sample surface at a constant rate for a predetermined test period. The test cycle is started by pushing the run switch, which is located on the scanner probe. The particle counter counts and reads out a number corresponding to the average number of particles per unit area. The process is usually repeated several times along adjacent surface areas, each time yielding a “test reading”.
An improvement of the technique disclosed in the '538 patent is one disclosed in U.S. Pat. No. 7,010,991, which is incorporated herein by reference for all purposes. The '991 patent describes a device for counting particles on a sample surface. The device includes a scanner probe having at least one opening for receiving particles from a sample surface, a particle counter for counting particles passed there-through, a conduit having a first end connected to the scanner probe and a second end connected to the particle counter, wherein the conduit includes first and second tubes, a sensor and a controller. The particle counter includes a pump for producing an airstream flowing from the scanner probe opening, through the first tube, through the particle counter, and back to the scanner probe via the second tube, for carrying the particles to the particle counter for quantitation and delivering an airstream flowing back to the scanner probe. The sensor measures a rate of flow of the airstream. The controller controls a speed of the pump in response to the measured rate of flow of the airstream to maintain the airstream at a constant flow rate while the particle counter quantitates the particles in the airstream.
The '991 patent further describes a device including a scanner probe having at least one opening for receiving particles from a sample surface, a conduit having a first end connected to the scanner probe and a second end terminating in a first connector, wherein the conduit includes first and second tubes; a particle counter, electronic indicia, and a controller. The particle counter counts particles passed there-through, and includes a port for receiving the first connector and a pump for producing an airstream flowing from the scanner probe opening, through the first tube, through the particle counter, and back to the scanner probe via the second tube, for carrying the particles to the particle counter for quantitation and delivering an airstream flowing back to the scanner probe. The electronic indicia is disposed in at least one of the first connector, the conduit and the scanner probe for identifying at least one characteristic of the scanner probe. The controller detects the electronic indicia via the port and first connector, and controls the particle counter in response to the detected electronic indicia.
The '991 patent further describes a device including a scanner probe having at least one opening for receiving particles from a sample surface, a particle counter for analyzing particles passed there-through, and a conduit having a first end connected to the scanner probe and a second end connected to the particle counter. The conduit includes first and second tubes. The particle counter includes a pump for producing an airstream flowing from the scanner probe opening, through the first tube, through the particle counter, and back to the scanner probe via the second tube, for carrying the particles to the particle counter and delivering an airstream flowing back to the scanner probe. The particle counter also includes a particle detector for counting the particles in the airstream coming from the scanner probe, a filter cartridge port through which the airstream flows after passing through the particle detector, and a filter cartridge removably connected to the filter cartridge port for capturing the particles in the airstream after being counted by the particle detector.
The efficiency of the above described particle counting devices can be classified as the number of particles extracted from the sample surface and captured/counted by the device, divided by the total number of particles on the sample surface. In order for a particle to be extracted, the air flow across the sample surface created by the scanner probe must be sufficient to overcome the adhesion force between the particle and the sample surface. One known problem of conventional scanner probes, however, is that as the airflow rate of the airstream is increased to attempt to better overcome the adhesion forces of more particles, more of the dislodged particles can be blown away from the scanner probe in which case they are never captured and counted by the device. This problem is called particle ejection, where particles dislodged by the scanner probe are ejected from the area under the scanner probe, where the particles cannot be captured and detected. Thus, merely increasing the velocity of airstream into the scanner probe can result in lower efficiency due to particle ejection, and therefore scanner probe efficiency cannot be fully maximized simply by increasing the velocity of the airstream. Because of particle ejection, there is a limit to the efficiency of these devices.